Most air operated tools of conventional construction have used springs to hold position or activate valves. This results in the disadvantage that these springs either wear or break and require frequent service of such equipment.
Conventional air tools include impact tools such as hammers and other devices to provide reciprocal motion. Such reciprocal motions are frequently such that the forward and the backward motion are more or less symmetrical. It has been desirable to have units where the forward and the backward motion are different such that the difference between the friction of initiation and the sliding friction can be used to move materials depending on the cycle of the strokes. Furthermore, air driven tools in most cases contained springs, which present a service problem since they are subject to wear and breakage.